The present invention relates generally to semiconductor integrated circuit (IC) technology. In particular, the invention relates to a unique semiconductor circuit used for wide-range low-dropout current regulation.
In known electronics designs, power control includes voltage or current regulation. One popular example that requires constant current control is the light-emitted-diode (LED) controller application. As a lighting source, an LED unit is required to work with a wide range of brightness, which is proportional with the forward current going through the LED unit. For this reason an LED unit 10 is often driven by a constant current regulator 12 as indicated in FIG. 1.
Many applications require multiple LED units 14, 16, 18 (FIG. 2) to achieve the required brightness and/or distribute the light over a larger area. When all the LED units are placed in series, the LED currents are the same, and the brightness of each LED is therefore also the same. However, as multiple LED units are added in series, the voltage required to drive the LED unit chain is increased. This often requires a step-up voltage converter in order to obtain a sufficiently large enough working voltage. Adding a step-up converter adds to the system cost and degrades efficiency and is not always the preferred solution. Hence there is often a need to drive multiple LED units, or multiple strings of several LED units in series, in a parallel manner. Unfortunately the forward voltage drop of LED units is not well controlled across temperature, LED color, or even from unit to unit and therefore placing LED units in parallel results in poor current matching in the LED units, or LED strings. Hence the preferred way to drive multiple LED units or LED strings is using a separate current regulator (for example a gain/attenuator block 20, 22, 24) in connection with error amplifiers 26, 28, 30 controlling output MOSFETS 32, 34, 36 for each of the parallel chains to ensure the current as seen at current sense resistors 38, 40, 42 through each LED unit 14, 16, 18 is tightly controlled, as indicated in FIG. 2.
In some LED lighting source applications the overall light color is generated by a combination of different LED color units, often Red, Green, and Blue (RGB). This allows control of the chromatic hue or while balance of the resulting light. Different color LED units often have different current requirements and working forward voltages, thus making tight current control of each of the LED colors important and the use of multiple current regulated outputs mandatory. The current in each channel can be independently controlled by adjusting the effective value of the current sense resistor and/or changing the amount of gain or attenuation applied to the non-inverting input of the Error Amplifier.
When there is sufficient supply voltage, there will be enough voltage to maintain the correct forward voltage for the LED unit (or units) and a large enough voltage headroom across the current regulating circuit for it to function correctly. (In this case, headroom can be defined as the drain to source voltage of the output MOSFET.) However, as the supply voltage decreases the voltage headroom across the current regulator output can eventually be reduced to a value that prevents the current level from being maintained. Then the current will start to decrease as the supply voltage is further decreased. The point at which regulation is lost will be dependent on the forward voltage of each LED unit, and since this is not well-controlled, the matching of the currents for the different LED units will be lost under low headroom conditions. As regulation is lost, the brightness of the various LED units will differ and unevenness will appear across the display and/or a change in color will also appear if RGB LED units are used. Both effects are undesirable and in current practice the LED units need to be switched off in time to prevent such effects occurring.
What is needed is a better way to sense and control output of an array of LED units.